The present invention relates to cellular networks and, more particularly, to methods of improving capacity utilization of a cellular carrier with integrated voice and data traffic through methods involving cellular network admission control, power control, and quality of service control.
Until recently, cellular telephone networks provided service for voice users only and did not address high-speed data applications such as electronic mail, facsimile, image transmission, and FTP transfer. Currently, network engineers are designing cellular networks, in particular Code Division Multiple Access (CDMA) networks, to carry integrated voice and data traffic. For example, the Telecommunications Industry Association (TIA) TR-45.5 cellular CDMA standard committee is developing CDMA networks that can support high-speed data (HSD) along with conventional voice telephony, described in "TR-45.5 Working Committee for CDMA Phase 1 High Speed Data (HSD) Stage 1 Services Description for HSD Services," October, 1996, which is hereby incorporated by reference. The systems proposed by TR-45.5, for both the 1.25 MHz CDMA channel and the broadband 5 MHz channel, achieve high data rates by implementing a multi-code architecture, which allocates several codes to a single HSD user for parallel transmission, in contrast with a variable gain CDMA system where HSD users use a single code to transmit at different data rates. This allows the user using m codes to transmit at a multiple rate of m times the nominal rate of a single code, e.g., 9.6 kbps. Such a multi-code architecture was proposed in I. Chih-Lin and R. Gitlin, "Multi-Code CDMA Wireless Personal Communications Networks," ICC '95 Conference Record, pp. 1060-1064, June, 1995, and further described in I. Chih-Lin, G. Pollini, L. Ozarow, and R. Gitlin, "Performance of multi-code CDMA wireless personal communications network, " Proceedings of IEEE Vehicular Technology Conference, pp. 907-911, 1995, which are hereby incorporated by reference.
A major issue in the design of a CDMA system with both voice and data users is the efficient use of system capacity. Capacity can generally be defined as the number of voice users and data users at different transmission rates (also referred to herein as different data classes) that can be supported by a CDMA channel with specified Quality of Service (QoS) attributes, specifically frame error rate (FER) and service availability probability (SAP), where the SAP is the probability that the FER does not exceed a predefined threshold. The QoS for a CDMA user is directly related to the ratio of the received bit energy to interference power. In conventional CDMA systems that carry only voice traffic, the FER and SAP targets are identical for all users, and therefore system capacity is maximized when all voice frames are received at the same power level. In a multi-class system, however, different users have different FER and SAP targets, and the relationship between capacity and QoS is more difficult to analyze for two reasons. First, the actual QoS experienced by a voice or HSD user depends on the activity of interfering users and the tightness of the algorithm controlling received power. Second, QoS targets are often "soft" for certain classes of data users (e.g., Internet access), and these users can accept a lower QoS in exchange for enhanced capacity.
Once the intricate relationship between capacity and parameters, such as QoS of users, the mix of voice and data users, their activity level, and their power levels, is established, control methods can be developed that control power allocation, QoS levels, and/or the activity levels of existing users to give additional users access to the system, thereby enhancing capacity utilization.
One previous effort to model CDMA network capacity is K. S. Guihousen, I. M. Jacobs, R. Padovani, L. A. Weaver and C. A. Wheatley, "On the capacity of a cellular CDMA system," IEEE Transactions on Vehicular Technology, vol. 40, pp. 301-312, May, 1991. This interference-based model addresses voice-only systems and assumes perfect power control and a uniform distribution of users in the cell. It has been shown that the interference resulting from HSD mobile users located close to cell boundaries and transmitting at a multiple of the basic rate is much higher than the interference generated by a uniform distribution of an equivalent number of mobiles transmitting at the basic rate (Z. Liu, M. J. Karol, M. E. Zarki, and K. Eng, "Interference issues in multi-code CDMA networks," PIMRC 1996, pp. 98-102, October, 1996).
It has also been shown that capacity limitations result from imperfect power control (e.g., A. J. Viterbi and A. M. Viterbi, "Erlang capacity of a power controlled CDMA system," IEEE Journal on Selected Areas in Communications, vol. 11, pp. 892-899, August, 1993; R. Cameron and B. Woemer, "Performance analysis of CDMA with imperfect power control," IEEE Transactions on Communication Theory, vol. 44, pp. 777-781, July, 1996; F. D. Priscoli and F. Sestini, "Effects of imperfect power control and user mobility on a CDMA cellular network," IEEE Journal of Selected Areas in Communication, vol. 14, pp. 1809-1817, December, 1996). One model with imperfect power control was studied in N. B. Mandayam, J. Holtzman, and S. Barberis, "Erlang capacity for an integrated voice/data DS-CDMA wireless system with variable bit rate source," Proceedings of PIMRC, vol. 3, pp. 1078-1082, 1995. This model, however, addresses only single code data users, and does not analyze a multi-code CDMA architecture.
The capacity of a CDMA system is also limited by the coverage area of handsets, which depends on the limitations on handset power. In a CDMA system based on the IS-95A standard, voice handsets have a peak output power of 200 mW. An HSD handset with identical power output transmitting at four times the basic rate outputs a maximum of 50 mW per code/channel. A proper analysis of CDMA system capacity should also consider the possible received power limitations for voice and data users.
It is desirable, therefore, to provide a method for detennining the capacity of a CDMA system with a mix of voice and multi-code high-speed data users with varying QoS requirements. It is more desirable to provide such a method that considers capacity limitations resulting from imperfect power control. It is also desirable to provide such a method that also considers handset power limitations. Having established such capacity estimation methods, it is ultimately desirable to provide methods for admission control to a CDMA network based on the determined relationship between system capacity and QoS, power allocation, and user activity level, thereby enhancing capacity utilization.